This invention relates to an optical disk driving apparatus on which information is recorded or reproduced by optical means while a disk-shaped record carrier is being rotated and, more particularly, to the control system of the apparatus.
FIG. 7 is a block diagram showing the control system of a prior art optical disk driving apparatus which is similar to an apparatus disclosed in Japanese Patent Application 156526/1986. FIG. 8 is a diagram of the signal waveform of a velocity detection circuit which detects a velocity signal when the spot of a light beam traverses the track of an optical disk. Referring to FIG. 7, numeral 1 indicates an optical disc record carrier in which information is to be recorded or has been recorded on concentric or spiral tracks at predetermined intervals. Numeral 2 is a light beam which is a medium for transmitting information to or from the optical disk. Numeral 3 is an optical head, numeral 4 is the carriage of the optical head, and numeral 5 is a head actuator which drives the carriage to move the light beam 2 in a direction traversing the track of the optical disk 1. Shown at numeral 6 is a tracking actuator which is mounted on the carriage 4. The tracking actuator 6 is furnished with a condensing lens for focusing the light beam 2 into a spot on the tracks of the optical disk 1 and is turnable in the moving direction of the head actuator 5. The tracking actuator is so constructed that the spot of the light beam 2 covers a predetermined number of tracks of the optical disk.
Numeral 7 denotes two photodetectors or sensors through which the quantity of light from the light beam 2 reflected from the optical disk 1 is converted into an electric signal that is delivered as an output. The electric signal which corresponds to the quantity of the reflected light from the light beam 2 falling on each of the sensor parts is output from the respective sensor part. A subtractive amplifier 11 receives the electric signals from the respective photodetectors 7 and produces a difference signal corresponding to the movement of the spot of the light beam 2 across the tracks of the optical disk 1. An additive amplifier 12 receives the electric signal from the respective photodetectors and produces a sum signal corresponding to the movement of the spot of the light beam 2 across the tracks of the optical disk 1.
A velocity detection circuit 13 detects a velocity signal when the spot of the light beam 2 traverses the tracks of the optical disk 1 from the difference signal produced by the subtractive amplifier 11. A direction detection circuit 14 detects a direction signal indicative of a direction in which the spot of the light beam 2 moves from the difference signal produced by the subtractive amplifier 11 and the sum signal produced by the additive amplifier 12, considering the phase relationship of the waveforms of the respective signals. Numeral 15 denotes a velocity direction designation circuit in which the direction signal received from the direction detection circuit 14 is combined with the velocity signal received from the velocity detection circuit 13, the resulting signal being provided as an output. On the basis of the difference signal produced by the subtractive amplifier 11, a pulse generator circuit 16 generates a pulse signal each time the spot of the light beam 2 traverses the track of the optical disk 1.
A track counter 17 counts the number of remaining tracks which the spot of the light beam 2 is to traverse on the optical disk 1 in accordance with an externally applied signal corresponding to the number N of tracks to be crossed to reach a target track. The pulse signal is delivered from the pulse generator circuit 15 and provides the count signal as an output. Shown at numeral 18 is a reference velocity generator circuit which receives the count signal from the track counter 17. Initially, the reference velocity generator circuit 18 determines and stores a reference velocity pattern corresponding to the number of the remaining tracks, namely, the number of the tracks to be crossed. Circuit 18 produces reference velocity signals successively as it receives the count signals from the track counter 17 corresponding to the gradual decrease of the number of the remaining tracks. A reference velocity direction designation circuit 19 combines an externally applied direction signal D indicative of the moving direction of the spot of the light beam 2 with the reference velocity signal received from the reference velocity generator circuit 18. Circuit 19 provides the resulting signal as an output. A velocity error detection circuit 21 receives the velocity signal from the velocity direction designation circuit 15 and the reference velocity signal from the reference velocity direction designation circuit 19 and compares them to provide a velocity error signal. Numeral 22 represents an amplifier circuit which amplifies the velocity error signal received from the velocity error detection circuit 21 and controls the velocity of the head actuator 5. Numeral 25 represents a tracking servo pull-in command circuit which receives the difference signal from the subtractive amplifier 11, the velocity signal from the velocity detection circuit 13, and the count signal from the track counter 17 and produces a position control command signal when the velocity of the spot of the light beam 2 has decreased below a predetermined velocity at a predetermined track of the optical disk 1 before the track target.
Numeral 26 indicates a tracking servo circuit which receives the difference signal from the subtractive amplifier 11 and the position control command signal from the tracking servo pull-in command circuit 25 and positions the spot of the light beam 2 on the target track of the optical disk 1 through the position control of the tracking actuator 6.
The prior art optical disk driving apparatus is constructed as described above, and the operation thereof will now be described. Track access control modes for positioning the spot of the light beam 2 on the target track of the optical disk 1 include a velocity control mode wherein the carriage 4 is driven by the head actuator 5 to move the spot of the light beam 2 in the direction traversing the track of the optical disk 1. In a position control mode, when the spot of the light beam 2 has decelerated below the predetermined velocity at the predetermined track of the optical disk 1, the tracking actuator 6 is controlled to stop so that the center of the spot of the light beam 2 and the target track are coincident.
First, in the velocity control mode, the signal corresponding to the number of tracks to be accessed (N) is externally applied to the track counter 17. Since the track counter 17 initially has no pulse signal from the pulse generator circuit 16, it regards the applied number of tracks to be accessed (N) as the number of remaining tracks and produces a count signal corresponding thereto. The reference velocity generator circuit 18 receives this count signal, determines and stores the reference velocity pattern in advance, and successively produces from track counter 17 reference velocity signals in accordance with the number of remaining tracks. The reference velocity direction designation circuit 19 combines an externally applied direction signal D indicative of the moving direction of the spot of the light beam 2 with the reference velocity signal and produces a resulting reference velocity signal. This reference velocity signal, including the direction, and the velocity signal, with the designated direction received from the velocity direction designation circuit 15, are applied to and compared by the velocity error detection circuit 21. The velocity error signal from circuit 21 is amplified by the amplifier circuit 22 to control the moving velocity of the head actuator 5.
In accordance with the reference velocity pattern, the head actuator 5 accelerates through a predetermined number of tracks, thereafter reaches a constant velocity, and decelerates when a predetermined number of tracks has been crossed. Due to the velocity control of the head actuator 5, the spot of the light beam 2 traverses the tracks of the optical disk 1 and moves to a target track. When a track is traversed, the quantity of the light beam 2 reflected from the optical disk 1 changes. When the quantities of the reflected light of the light beam 2 impinging on the respective sensor parts of photodetector unit 7 have changed, these sensor parts convert the changes of the light quantities into the electric signals. The subtractive amplifier 11 and the additive amplifier 12 receive the electric signals from the two divided photodetectors 7 and supply the difference signal (FIG. 8(b)) and the sum signal (FIG. 8(a)), respectively.
The periods of the difference signal and the sum signal are determined by the velocity at which the spot of the light beam 2 moves across the tracks of the optical disk 1. In the waveform of the difference signal, the zero point of every cycle indicates that the center of the track of the optical disk 1 has coincided with the center of the spot of the light beam 2. The velocity detection circuit 13 receives the difference signal from the subtractive amplifier 11, detects the velocity signal (FIG. 8(d)) from the period thereof, and produces this velocity signal as an output. The direction detection circuit 14 receives the difference signal from the subtractive amplifier 11 and the sum signal from the additive amplifier 12 and produces a direction signal indicative of the moving direction of the spot of the light beam 2 on the basis of the phase relationship between the waveforms of the received signals.
In the velocity direction designation circuit 15, the direction signal received from the direction detection circuit 14 is combined with the velocity signal received from the velocity detection circuit 13. In addition, the pulse generator circuit 16 receives the difference signal (FIG. 8(b)) from the subtractive amplifier 11 and generates a signal transition, i.e, rectangular pulse signal, (FIG. 8(c)) at the zero crossing point of every cycle of the difference signal waveform, indicating that the spot of the light beam 2 has traversed a track. The track counter 17 receives those pulses and successively subtracts them from the externally applied signal corresponding to the number of tracks to be accessed (N). Counter 17 thereby determines the number of tracks remaining to be traversed by the spot of the light beam 2 and produces a count signal containing that information.
The reference velocity generator circuit 18 receives th count signals and successively generates reference velocity signals in accordance with the reference velocity pattern previously determined and stored. In the reference velocity direction designation circuit 19, the externally applied direction signals D indicative of the direction in which the spot of the light beam 2 is to move is combined with the reference velocity signals, the resulting signal being generated by circuit 19. The velocity error detection signal 21 compares the velocity signal for the designated direction from the velocity direction designation circuit 15 with the reference velocity signal for the designated direction from the reference velocity direction designation circuit 19 and produces a velocity error signal. The velocity error signal is amplified by the amplifier circuit 22 to control the subsequent moving velocity of the head actuator 5. The tracking servo pull-in command circuit 25 receives the difference signal from the subtractive amplifier 11, the velocity signal from the velocity detection circuit 13, and the count signal from the track counter 17. If the moving velocity of the spot of the light beam 2 is lower than a predetermined velocity when the spot has come to a predetermined track before the target track, for example, one track before the same, the tracking servo pull-in command circuit 25 produces a position control command signal, whereupon the velocity control mode shifts to the position control mode.
In the position control mode, the tracking servo circuit 26 receives a position control command signal from the tracking servo pull-in command circuit 25 and the difference signal from the subtractive amplifier 11 and produces a tracking signal, taking into account the phase of the difference signal waveform from the subtractive amplifier 11. Thus, the tracking actuator 6 is controlled and is stopped with the center of the spot of the light beam 2 coincident with the center of the target track to finish the so-called "track pull-in". Thereafter, information is recorded and reproduced while the spot of the light beam 2 is following the target track of the rotating optical disk 1.
With the prior art optical disk driving apparatus constructed and operated as described above, when information is recorded in the form of a pit on the track or when there is a defect on a track or between tracks, the quantity of light reflected from the corresponding part is momentarily zero when the spot of the light beam 2 traverses the track or tracks of the optical disk 1. As a result, intermittent electric signals are delivered from the respective sensor parts of the photodetectors 7. Upon receiving the intermittent electric signals, the subtractive amplifier 11 and the additive amplifier 12 produce a difference signal having a momentary omission. The sum and difference signals are as respectively shown in FIGS. 8(b) and 8(a). When the difference signal having the momentary omission enters the pulse generator circuit 16, the number of the pulse signals to be delivered to the track counter 17 increases and an erroneous number of remaining tracks is counted (FIG. 8(c)). When the difference signal enters the velocity detection circuit 13, an erroneous velocity signal is detected (FIG. 8(d)). Therefore, the prior art has had a problem in lengthening the track access time required for positioning the spot of the light beam 2 on the target track of the optical disk 1.